smtp problem

hey there,
i've created an email box on one of the accounts,
with this user i try to send bulk mails (it's not spam, it's a letter for my users)
after it sends 10000 mails it stops sending and tells me "We do not authorize the use of this system to transport unsolicited, and/or bulk e-mail."
how can i prevent it?
i need to send 300,000 mails every day
any suggestions?